RF Practice: better to know
create @2014/6/25
something important to know when you design/debug/thing about a RF circuit, like , the SRF of a inductor is? inductance of a component lead? capacitance of a 1/4 watt resistor?
Theory
pure resistance in parallel with a reactance, the apparent resistance vary with frequency. If the dominant parallel element is capacitive, the effective resistance will fall with frequency. If the dominant parallel element is inductive (as it is a wire or low-resistance conductor) the effective resistance will increase with frequency.
*skin-depth in a conductor is proportional to the square-root of the resistivity (ρ)
since the reactance of an inductor (XL=2πfL) increases with frequency, and the RF resistance increases with frequency(Proximity effect), the Q of the coil (being the reactance divided by the RF resistance) varies relatively slowly as the frequency is changed <<G3YNH From Transmitter to Antenna>>
Selecting components
*skin-depth in copper (for example) being only 48 μm (0.048 mm) at 1.9 MHz. <<G3YNH From Transmitter to Antenna>>
*a tubular conductor is just as good as a solid wire, and may offer a considerable saving in weight and cost when a large diameter is required. <<G3YNH From Transmitter to Antenna>>
*silver-plated copper wire or tubing is the best commonly available conductor for radio applications, with plain copper (bare or enamel coated) coming a close second. if the silver is allowed to tarnish, the RF resistance will eventually exceed that which can be achieved using plain-copper that has been subjected to the same environmental conditions . increase in losses for not using silver can often be offset simply by increasing the wire diameter. <<G3YNH From Transmitter to Antenna>>
Bunch-wire wire does reduce the effective resistance significantly at very low radio-frequencies, but its usefulness fades as the frequency is increased
As the frequency is increased further, current will not even venture into the valleys between the conductors, at which point the RF resistance will be much the same as that obtained by using a single wire of the same overall diameter although bunch-wire is used for coils and interconnections in switched-mode power converters (tens of kHz), it is not much used in HF radio applications.
<<G3YNH From Transmitter to Antenna>>
Litzendraht or "Litz wire" gives an improvement in high-frequency performance in comparison to bunch-wire because it is woven in such a way that, in a given length, every conductor has an equal chance of appearing in the outside layer of the bundle (i.e., it is braided or plaited). In this way, no conductor is completely buried, and every conductor is forced to carry current. The use of litz wire is generally beneficial between about 50 kHz and 3 MHz [14][15], but offers only marginal advantage at the higher end of this range (i.e., on the 160m band) <<G3YNH From Transmitter to Antenna>>
Typical parasitic value
Maximum enclosed area: Maximum stray inductance, minimum stray capacitance.
Minimum enclosed area: Minimum stray inductance, maximum stray capacitance.
The #24 AWG wire typically used for component leads has an inductance on the order of 20 nH per inch., 18 awg had 25nh per inch. <<arrl hand book 2011 Chapter 5>>( what is the inductance of a piece of wire is almost tell you nothing, cause wire inductance domain by external current loop. <G3YNH From Transmitter to Antenna> )
½Watt metal-film resistor. body-length of 9 mm, a body diameter of 2.5 mm, and lead diameter of 0.8 mm, given about 13nH inductance , @ several Mhz. <G3YNH From Transmitter to Antenna>
A wire-ended ½Watt carbon-film resistor will typically have a shunt capacitance in the range 0.2 - 0.5 pF, and a metal film resistor typically 0.2 - 0.6 pF [20].
100 Ω resistor is very nearly a perfect component over the entire HF spectrum, and a 1 kΩ resistor is not too bad. Our cheap 10 kΩ resistor however, has an impedance of about 5000-j5000 Ω at 32 MHz, and resistors of higher value than that are basically just poor-quality capacitors. <G3YNH From Transmitter to Antenna> (this does not mean that high value resistors are of no use at radio frequencies; a 1 MΩ resistor providing DC bias to the gate of a JFET, for example, will still do its job, but from a radio-frequency point of view, it simply adds about 0.5 pF of stray capacitance to the amplifier input impedance. only the 100 Ω resistor (in this set of examples) is capable of displaying a positive reactance when the inductive reactance is added. In all other instances, the impedance is vastly dominated by the capacitance, and the circuit inductance is effectively immaterial.) [NOTE YOUR SWR RESISTOR DIVIDER/ resistors in parallel ]
Typical SRF
Toroidal Inductors FT-50-61, 32turn had around 1pF capacitance, 5T around 3pF, 10T 2pF, 20T 1.xpf.
reference:
<ARRL hand book>
<G3YNH From Transmitter to Antenna>